Final answer:
The element with the least energy required for the reaction is sodium (Na). The correct option is b.
Step-by-step explanation:
The energy required for a reaction can be related to the ionization energy, which is the energy required to remove an electron from an atom. The element with the least energy required for the reaction would have the lowest ionization energy.
Looking at the listed elements, the element with the lowest ionization energy would be (b) Na (sodium). Sodium is located in the same group as (a) Ga (gallium) and (d) K (potassium), but it has the lowest atomic number among them, which indicates a lower ionization energy.
The energy required for the reaction would be least for (b) Na (sodium).